1. Field of the Invention
The present invention relates generally to fixed cutter earth boring drill bits and, more particularly, to PDC type drill bits having novel integrally formed wear and erosion resistant surfaces.
2. Description of the Related Art
There are two basic types of earth boring drill bits commonly used to form the boreholes in the earth for mineral exploration and recovery. The first utilizes one or more rolling cutters mounted upon a bit body. There are typically several rows of cutting teeth on each cutter. When the bit body is rotated and weight is applied, the teeth on the cutters engage the earth causing the cutters to rotate. As the cutters rotate, the teeth are sequentially pushed into the earth effecting a drilling action. These bits are commonly known as rolling cutter drill bits or rock bits.
The second type of earth boring bit, and the subject of the present invention, utilizes cutting elements fixed upon the body of the bit. These bits are also rotated, and when weight is applied, the cutting elements are pushed into, and dragged through the earth. This dragging action causes earth removal by shearing.
There are different fixed cutter bit designs for different drilling applications. For example, a high bladed steel bit (often called a fishtail bit) may be suitable for rapidly drilling through very soft soils and formations, while a polycrystalline diamond compact (PDC) bit may be used to drill through harder rock formations. For very hard and tough rock formations, infiltrated tungsten-carbide matrix bits with natural diamond cutting elements are used. These are typically called diamond or natural diamond bits.
As a general rule, bits that are able to drill rapidly through soft formations cannot penetrate the harder formations and, similarly, bits that are able to drill through harder formations are not aggressive enough to economically drill through softer formations. Thus, when drilling deep wells through many different types of rock and soil, bits may have to be changed many times in response to wear or in response to changing soil conditions.
Common to all types of earth drilling bits is a means to flush the drilled earth away from the cutting interface and transport it to the surface. For shallow boreholes, air is a suitable flushing fluid. However, for the deep boreholes commonly drilled for the exploration and production of oil and gas, the flushing fluid is typically a liquid. Because of its color and consistency, this liquid has come to be known as drilling mud. Although the type of drilling fluid may vary, it typically the contains abrasive elements, and it is usually pumped through nozzle orifices on the drill bit, typically at a rate of about 250 to 500 feet per second.
In rolling cutter drill bits the primary role of drilling mud is to clean the bottom of the boreholes and transport the cuttings to the surface. In fixed cutter drill bits with diamond cutting elements, however, the drilling mud has the added critical role of cooling the diamonds. Clearly, diamond, and other suitable forms of superhard materials, are much harder than the earth formations being drilled, so theoretically these materials should not exhibit any wear. However, it is also apparent from examination of used bits that the superhard cutting elements do degrade. It was found that the degradation of the superhard cutting elements was caused, at least in part, by the high temperatures generated at the cutting face from the friction of scraping the rock. In order to minimize the degradation of the cutting faces, they must be cooled. For maximum cooling (and therefore minimum degradation), it is desirable to have the drilling fluid impinge directly on the cutting elements. However, PDC bits generally have exposed steel or infiltrated matrix surfaces adjacent to the diamond cutting elements, which can rapidly erode in the high velocity, abrasive laden stream of drilling fluid. There are numerous patents which show high velocity drilling fluids directed upon superhard cutting elements in steel bodied PDC bits, as shown, for instance, in U.S. Pat. Nos. 4,484,489; 4,907,662; 4,974,994; 4,883,136; 4,452,324; 4,303,136 as well as many others. Unfortunately, it is not possible to direct the flow in this manner without causing severe erosion of the surface adjacent to the cutting elements.
For this reason, the nozzle orifices on PDC drill bits are oriented such that high velocity drilling fluid does not directly impinge the diamond cutting elements. Thus, although directing the drilling fluid at the diamond cutting elements on PDC bits would provide better cooling and longer life, commercial drill bits do not incorporate this feature because of erosion. Instead, the nozzle orifices typically direct the drilling fluid toward the formation at the bottom of the hole, and the splash is used to clean and cool the superhard cutting elements. As a consequence, typical PDC bits do not perform well where very high cutting element face friction is present, such as in hard rock drilling.
In addition, where soft, sticky formations are encountered, such as shales with high clay content, the hydraulic action of conventional PDC bits is sometimes inadequate to clean the cuttings away from the bit body and cutters resulting in a phenomenon known as bit balling. Most drilling applications allow for between 100 hydraulic horsepower (HHP) and as much as 800 HHP at the bit. Optimizing the use of this significant source of energy to clean and cool the bits requires proper orifice size selection and proper placement of the nozzles, including optimum orientation.
In the past, there have been many different attempts to address the erosion problem described above. The tried and true method to obtain erosion resistance is to apply welded hardmetal in thick layers to the surface of the cutting face. This is the most common form of wear resistant material in use today for steel bodied PDC bits. Unfortunately, welded hardmetal can crack as the blades of the PDC bit bend in response to the drilling loads. Once a crack starts, the impinging drilling fluid quickly erodes the exposed, soft underlying steel layer. Applying welded hardmetal is typically a hand applied process and it is difficult to apply to the sides and bottom of the channels on the cutting face of PDC bits. Because it is a manual process, it is also subject to variation based on human and environmental factors. Once the welded hardmetal is applied, it is generally so thick and uneven that it affects the hydraulic flow of the flushing fluids. The swirls and flow eddies in the wake of these thick, rough layers can make the erosion problem even worse. Finally, the temperature caused by the welding process not only affects the heat treatment of the steel PDC bit bodies, it can also cause the bodies to warp and even crack due to the thermal stresses and can have a deleterious effect on the diamonds themselves.
Another approach to erosion resistance is shown by Radtke in U.S. Pat. No. 4,396,077, herein incorporated by reference. Radtke describes a thick tungsten carbide coating applied to the cutting faces of PDC bit bodies with a high velocity plasma arc flame spray process. This process was considered an improvement over the conventional high velocity flame spray processes known at the time. Unfortunately, the problem with this and all other flame spray type coating processes is that the sprayed particle stream must impinge nearly perpendicular to the surface to be coated to make the coating adhere to the cutting face of the bit body. Although sprayed coatings can provide good erosion protection on some areas of the bit, the coating does not adhere well to the vertical surfaces normal to the cutting face. PDC bits usually have channels formed in the cutting face for the high velocity flushing fluid. Since these channels usually have vertical walls, spray type coatings to not provide adequate erosion resistance in these areas of the bit. Also, the discharge nozzle on the flame spray apparatus is generally located some distance away from the surface being coated. The irregular features on the cutting faces of most PDC bits cause `shadows` which block the spray path, preventing direct impingement by the spray. These limitations greatly reduce the effectiveness of the flame spray processes for producing wear and erosion resistant coatings on PDC bits.
Natural diamond bits (also called diamond bits) are very old in the drilling industry and provide an alternate way of addressing the wear and erosion problems of fixed cutter drill bits. This type of fixed cutter drill bit is made in an infiltration process. In this process, natural diamonds or other very hard fixed cutting elements are inserted onto cavities in a mold. Powders of highly wear and erosion resistant materials (typically including tungsten carbide) are then packed into the mold, and an infiltrate, typically a copper alloy, is placed in contact with the powders. The mold with the powders, cutting elements, and infiltrate are all placed into a furnace and heated to the melting point of the infiltrate. The melted infiltrate fuses the diamonds and powders into a solid mass. This process produces a unitary body of infiltrated tungsten carbide and fixed cutting elements with improved wear and erosion resistance. By way of example, an early diamond bit design is disclosed in U.S. Pat. No. 2,371,489.
It is also possible to form pockets in an infiltrated cutting face and later attach polycrystalline diamond cutters, as shown in U.S. Pat. No. 4,073,354, providing a somewhat more aggressive cutting structure than traditional diamond bits.
Unfortunately, infiltrated bits are expensive to manufacture. Each bit must be cast in a mold in a very labor intensive process.
Infiltrated bit structures are also weak in bending, so the blade height achievable with an infiltrated product is limited by the intrinsic strength of the material in bending. Therefore, these relatively shorter blades do not penetrate the earth as aggressively as the extended cutting faces of steel PDC bits. As a result, infiltrated bits do not provide the very high (and desirable) rates of penetration of PDC bits.
Finally, because the infiltrated products use a relatively soft copper based infiltrate to bind the tungsten carbide together, the infiltrated product can also be subject to erosion as the fluid stream attacks the copper binder, weakening the matrix and allowing tungsten carbide to be loosened from the body. The infiltrated design provides some erosion improvement over steel, but is still subject to all the limitations described above.
There are also numerous bit designs which are derivatives of either the infiltrated bit process or the coated steel process used in PDC bits. For example, in U.S. Pat. Nos. 4,554,130; 4,562,892; and 4,630,692, all herein incorporated by reference, a cladding process is disclosed for making a PDC type bit with a layer of wear and erosion resistant material. In these patents, a steel blank is coated with a thick layer of powders, the assembly is heated and then transferred to a press where the powders are fused to the steel surface under temperature and pressure with the aid of a ceramic or graphite pressure transfer medium. The layer must be thick, for it must contain a binder along with the wear resistant powder as it is compressed in the press. Although PDC type bits are shown and described in these patents, it is impractical to clad the vertical surfaces as shown. This is because the movement of the pressure transfer media tends to scrape the powders from the vertical steel surface as the press closes. Also, because the steel body itself is incompressible, the pressure transfer media will not be able to move in a manner which allows for an even pressure distribution.
The end product of the above described cladding process has many of the same deficiencies as the flame spray coatings previously described, in that the vertical surfaces will not have adequate erosion protection.
Another derivative process that is similar to infiltration is disclosed in U.S. Pat. No. 4,499,795. This patent describes a bit formed by a molten steel casting process wherein a tungsten carbide powder coating is applied to the walls of the casting mold and molten steel poured in. The patent does not disclose how the tungsten carbide is able to retain its wear resistant properties after a prolonged time at the temperature of molten steel. Similarly, it is not disclosed how the powders stay adhered to the walls of the mold as the very turbulent flow of steel is introduced. Nor does the patent disclose how to prevent excessive surface cracking of the coating as it shrinks and cools. The problematic nature of this process is the likely reason that it is not in commercial use today.
In summary, it would be desirable to have a fixed cutter drill bit with superhard cutting elements that can drill the soft formations of the earth at high drilling rates of penetration, and at the same time drill the hard intermingled layers of earth formations without significant cutter degradation. There are also many drilling applications where it would be desirable to have the aggressive behavior of a high-bladed steel bit coupled with the erosion and abrasion resistance of a matrix body bit. Furthermore, it would be desirable to provide a fixed cutter drill bit having an overlay that exhibits erosion resistant qualities superior to those of traditional hard faced steel drill bits while maintaining the strength and toughness of a steel body. This greater erosion resistance would permit more aggressive fluid hydraulics in which fluid nozzle orifices could be aimed directly at the blades to facilitate cooling of the diamond or other superhard material layer and enhance removal of the drilled cuttings without reducing the life of the drill bit below a commercially acceptable level.